posted on 2024-02-27, 13:03authored byAaron
M. Chalifoux, Cody Nizinski, Miguel Cisneros, Travis Tenner, Benjamin Naes, Kimberly N. Wurth, Erik Oerter, Michael Singleton, Gabriel J. Bowen, Luther W. McDonald
Within the front end of the nuclear fuel cycle, many
processes
impart forensic signatures. Oxygen-stable isotopes (δ18O values) of uranium-bearing materials have been theorized to provide
the processing and geolocational signatures of interdicted materials.
However, this signature has been minimally utilized due to a limited
understanding of how oxygen isotopes are influenced during uranium
processing. This study explores oxygen isotope exchange and fractionation
between magnesium diuranate (MDU), ammonium diuranate (ADU), and uranyl
fluoride (UO2F2) with steam (water vapor) during
their reduction to UOx. The MDU was precipitated
from two water sources, one enriched and one depleted in 18O. The UO2F2 was precipitated from a single
water source and either directly reduced or converted to ADU prior
to reduction. All MDU, ADU, and UO2F2 were reduced
to UOx in a 10% hydrogen/90% nitrogen
atmosphere that was dry or included steam. Powder X-ray diffraction
(p-XRD) was used to verify the composition of materials after reduction
as mixtures of primarily U3O8, U4O9, and UO2 with trace magnesium and fluorine
phases in UOx from MDU and UO2F2, respectively. The bulk oxygen isotope composition
of UOx from MDU was analyzed using fluorination
to remove the lattice-bound oxygen, and then O2 was subsequently
analyzed with isotope ratio mass spectrometry (IRMS). The oxygen isotope
compositions of the ADU, UO2F2, and the resulting
UOx were analyzed by large geometry secondary
ion mass spectrometry (LG-SIMS). When reduced with steam, the MDU,
ADU, and UO2F2 experienced significant oxygen
isotope exchange, and the resulting δ18O values of
UOx approached the values of the steam.
When reduced without steam, the δ18O values of converted
ADU, U3O8, and UOx products remained similar to those of the UO2F2 starting material. LG-SIMS isotope mapping of F impurity abundances
and distributions showed that direct steam-assisted reduction from
UO2F2 significantly removed F impurities while
dry reduction from UO2F2 led to the formation
of UOx that was enhanced in F impurities.
In addition, when UO2F2 was processed via precipitation
to ADU and calcination to U3O8, F impurities
were largely removed, and reductions to UOx with and without steam each had low F impurities. Overall, these
findings show promise for combining multiple signatures to predict
the process history during the conversion of uranium ore concentrates
to nuclear fuel.